1. Field of the Invention
The present invention concerns a superconductor and in particular a superconductor protected against partial transition.
2. Description of the Prior Art
The meaning of the terms "superconductor", "strand" and "filament" will first be explained.
The term "superconductor" refers to an assembly of separable members which in this context are the "strands". The strand is the smallest separable component. A superconductor strand is formed from superconductor "filaments" embedded in a metal matrix. The central strand or the central core is a metal composite.
The "filament" is a homogeneous subset of the composite strand.
There are various known superconductor configurations.
In the case of multistrand superconductors, the superconductor strands may be twisted around a central strand which is a superconductor or stainless steel or copper alloy, possibly with an insulating jacket.
European patent No 0037544 describes another kind of flat multistrand superconductor configuration having a central core in the form of a copper strip provided with an insulating "separator".
It is known that superconductors can undergo partial transitions either accidentally or under specific operating conditions; these "partial transitions" are changes of a limited area of the superconductor to the resistive state; this area is initially very small and then expands due to heating as a result of the Joule effect. The superconductor may be damaged by the rapid and intense heating and the high voltages which are produced across the transition area.
To moderate the effects of partial transitions the superconductor strands are usually provided with a matrix comprising a high proportion (up to 80% or even 90%) of a metal that is a good superconductor, such as copper or aluminum; steps are then taken to detect the transition and to reduce the current as quickly as possible. The reaction time and the time needed to reduce the current are sometimes too long for it to be possible to avoid overheating or overvoltages likely to damage the strands.
In the article in IEEE TRANSACTIONS ON MAGNETICS, Vol.MAG.17 No 5, September 1981: "High sensitive quench detection method using an integrated test wire", there is described a superconductor protection device comprising a strand or a central core in copper-nickel alloy on which is provided a voltage detector associated with signal processing means to introduce into the superconductor circuit resistances to reduce the current.
The complexity of a protection device of this kind multiplies the risk of failure.
An object of the present invention is to avoid the disadvantages and to enable simpler and more effective protection of superconductors.